Compression Problems
Good compression is vital to the operation of the engine. If there is not enough compression
the engine will be hard to start and won’t start at all, or idle poorly and have low power. Compressions
are quick and easy to test with the right tools.
Compression can be low because of damaged or worn bores, pistons or rings, blown head gasket,
corroded head, loose spark plugs, valves that are damaged or not sealing and incorrect valve clearances.
A weak battery, dirty battery terminals, corroded battery leads, faulty solenoid, worn or dirty starter motor,
a slow pull on the starter cord or a starter cord that is too short will all produce low compression.
When checking compressions on a 2 stroke outboard, you are only testing the very last section of the up stroke of
the piston. There could be damage below the section where compression is started.
2 stroke outboards require crankcase pressure to transfer the charge of fuel and air from the crankcase
to the combustion chamber. This crankcase pressure is developed by the down stroke of the piston.
A compression test will not test the down stroke.
Compressions variations between the lowest compression and the highest compression
on a multi cylinder outboard should be within about 10 - 15%.
It is possible to have good compression and still have damaged pistons. A visible
inspection of the bores and pistons should be performed where possible.
You may need to take your outboard to a mechanic for a compression test if you suspect
the compression is low.
- Contact Cairns Outboard Service for your outboard service requirements.